The invention relates to electrostatography and more particularly to a method and apparatus for applying a loading force to a roller used in image transfer.
In U.S. Pat. No. 6,075,965, issued Jun. 13, 2000, in the names of Tombs et al (the contents of which are incorporated herein by reference) there is disclosed an electrophotographic reproduction apparatus wherein a series of roller separation image forming stations are arranged so that a receiver sheet may be conveyed from station to station to receive color separation images in transfer so that the images arc superposed onto each other to form a composite multicolor image. In the aforesaid application at each color separation station a compliant intermediate transfer drum or roller (ITR) has an electrically conductive core of, for example, aluminum, a relatively thick (1-20 mm) compliant blanket layer is around the core and a relatively thin (2 micrometers-30 micrometers) hard overcoat layer surrounds the blanket layer. The Youngs modulus of the blanket layer is preferably between 0.1 MPa and 10 MPa, and the blanket layer has a bulk volume electrical resistivity preferably between 107-1011 ohm-cm. The Young""s modulus of the overcoat layer is preferably greater than 100 MPa. The ITR forms a nip under pressure with a photoconductive (PC) drum. An electrical bias is impressed upon the ITR of suitable level and polarity to urge a developed toner image on the PC drum to transfer to the surface of the ITR. A receiver sheet is then moved into a second nip between the ITR and a paper transfer roller (PTR) in a timed or registered condition to receive the image in transfer from the ITR. An electrical bias of appropriate level and polarity is provided to the PTR to urge transfer of the toner image to the receiver sheet.
Heretofore, a PC drum and ITR were urged together by controlling the separation of their respective axes and establishing a predetermined interference in their respective radii of which interference is accommodated by the compliancy in the ITR blanket layer. However, this solution is not desirable because accommodation must be made when the machine is not producing prints to separate the PC drum from the ITR to avoid set forming in the ITR. It is, therefore, an object of the invention to provide an improved method and 15 apparatus for establishing an engagement between a pair of drums or rollers in an image transfer relationship that is relatively stable and insensitive to force changes in a system. While load may vary due to inherent xe2x80x9crun-outxe2x80x9d in the drums, the run out can be minimized through manufacture of the drums and as described herein force variations otherwise present can be inexpensively reduced.
It is a further object of the invention to provide a method and apparatus for ensuring that the engaged position is very repeatable between disengagement and reengagement.
In accordance with the first aspect of the invention there is provided in a printing apparatus a method of applying a loading force to a first roller assembly to move a first roller into nip engagement with a second roller of a second roller assembly, the first roller assembly having a first fiducial element for locating the first roller and the second roller assembly having a second fiducial element complementary to the first fiducial element for locating the second roller, the first roller assembly being supported by a hook and pin connection engagement that supports a carnage which carriage supports the first roller assembly for rotation about an axis external to the first roller, the method comprising:
applying a loading force to the first roller assembly along a line which in a planar diagram of the first roller and load force passes between the center of gravity of the combination of the first roller assembly and the carriage and the first fiducial element to create a moment about the axis so that the carnage and the first roller assembly rotate together towards contact of the first fiducial element with the second fiducial element; and
continuing to apply the loading force to cause the first fiducial element and the second fiducial element to nest together with the hook and pin arrangement being clear of engagement.
In accordance with a second aspect of the invention, there is provided a printing apparatus comprising:
a first roller assembly including a first roller and a carnage for supporting the first roller assembly;
a second roller assembly including a second roller; the first roller assembly having a first fiducial element for locating the first roller and the second roller assembly having a second fiducial element complementary to the first fiducial element for locating the second roller;
a hook and pin connection engagement that supports the carriage which carriage supports the first roller assembly for rotation about an axis external to the first roller;
a loading force applicator that applies a loading force to the first roller assembly along a line which in a planar diagram of the first roller and load force passes between the center of gravity of the combination of the first roller assembly and the carriage and the first fiducial element to create a moment about the axis so that the carriage and the first roller assembly rotate together towards contact of the first fiducial element with the second fiducial element; and
and wherein upon continued application of the loading force causes the first fiducial element and the second fiducial element to nest together with the hook and pin arrangement being clear of engagement.